'Excuse me, are you Simon Stephens?' He is sitting on the front steps of Black's club, in Soho's Dean Street. His pose is a bit like Rodin's The Thinker - hand on chin. He is on his mobile, talking concentratedly, dressed in black - a sort of uniform. He could be a waiter or a theatrical usher. But I am guessing that I have the right man. I have just overheard him say he can't take on any more work - and that clinches it. For Stephens is in overdrive. To say that he is 'in demand' would be putting it mildly. 'Pornography', about the London bombings has just finished a successful run at The Tricycle in Kilburn, 'Sea Wall', a tragic monologue - a Father's Tale - has been at the Edinburgh Festival and his new play 'Punk Rock' is about to open at the Lyric Hammersmith (it is also a debut production from Sean Holmes as the theatre's new artistic director with Simon Stephens as his associate). It is true: Stephens does not have much room for manoeuvre.

He owns up to being himself at once - and scrambles to his feet. He is tall, good-looking with hair that seems under attack (all that earlier thinking). He seems to have the combined energy of several people in one body. I like his punchy, engaged manner. At different moments, I notice him throw his arms wide, as if someone had scored a goal and use both forefingers to add double emphasis to his points. If you were to guess what sort of plays he wrote from his demeanour, you would plump for dynamic comedy. But you would be wrong. His writing can entertain but it is darkness that draws him in.

Stephens's first success was with 'On the Shores of the Wide World' which won an Olivier award, in 2005, for 'best new play'. The following year 'Motortown' at the Royal Court, about the Iraq war, got admiring reviews. But then came his most celebrated play 'Harper Regan', directed by Marianne Elliott, at the National, in 2008. On the surface, the subject could not have been more drab. It was about a 41-year-old, living in Uxbridge, visiting her dying father in Stockport. But it was also a portrait of an England charged with secrets - and, as Michael Billington put it, the work of an accomplished playwright who understood the English curse of 'emotional evasion.'

Stephens's new play, 'Punk Rock', set in a fee-paying school in Stockport, is evasive too - the title, at least, is a decoy, does not prepare you for the play. But Stephens gives the crux of it away when he explains that it was the massacre at Columbine High School that first drove him, an event that has left its 'scar over the beginning of the 21st century'. On the page, the play reminded me of Alan Bennett's 'The History Boys' and also of Lionel Shriver's novel 'We Need to Talk about Kevin' but its tense rhythms are all its own. Stephens is keenly aware that 'What happened at Columbine and in Iowa State and in Germany could happen here.' As a writer, he is too close for comfort - that is his gift. It is important to him that his version of a school shooting should take place not only in this country but in a middle-class, fee-paying school. He argues against the reflex, especially in theatre, to consider violence in a 'working-class and often black, urban context.' The truth is that violence is 'pervasive.' And middle-class audiences will not be able to distance themselves from Stephens's terrible subject.

He describes his schoolboy assassin as 'romantic, violent, funny, charming and ironic - suffering from an increasing dislocation from himself.' For Stephens, there is no such thing as a villain. It is what makes his work dangerous, persuasive, conspicuous. We must make sense of a charming schoolboy assassin and - in 'Pornography' - of terrorists that we are asked to see as human beings. He is interested in human complexity and asks for an acceptance that good and evil are not always distinct, that black and white fuse. And what about the punk music (each scene begins with a blast)? How does that fit in? 'Punk rock is mistakenly considered working class but it more often came out of art school than housing estates. It is the musical manifestation of the anger and frustration a lot of middle-class kids feel. It cuts to the quick of that energy.'

All the teenagers in the play have something of the misfit about them. It is a 'murderously difficult period in most peoples' lives', Stephens says - and includes himself: 'I wasn't as clever or as rich as the kids I describe. As a teenager, I had immense periods of misery. If it hadn't been for the Smiths (his devotion to the band was absolute) I would have been an even more unhappy person.' It is hard to believe in this - his Northern accent sounds so upbeat and he is so full of sunshine. He grew up in Stockport, outside Manchester, the son of the managing director of a retail distribution company and a primary school teacher . He went to an all-boys comprehensive school - not much like the school he describes in the play. He is 38 now - and grew up during the Eighties when British television drama ruled (Dennis Potter dubbed the BBC the 'real National Theatre'). Stephens's teens were 'defined by Potter and Alan Bleasdale'. Later, at York (where he read History), the 'most attractive girls were aspiring actresses' and out of a 'pathetic and usually fruitless attempt to charm them' he would watch them in Chekhov and Ibsen. But there it was cinema, not theatre, that gripped him. Films such as 'Blue Velvet' and 'Taxi Driver' inspired him to write plays: 'I wondered what would happen if someone could write something for the theatre with the visceral power of those films.' He remains a film obsessive, adding, with disarming insistence, that 'Punk Rock' is indebted to Lindsay Anderson's 'If' and Gus Van Sant's 'Elephant' (about Columbine).

It is not surprising, given Stephens's zest for what he does, that he has been a natural choice for theatres wising to acquire a dramatist. He was resident dramatist at the Royal Court in 2001, a tutor on the Royal Court's Young Writers Programme between 2001 and 2005 and the first resident dramatist at the National. He has also taught in prisons. What has teaching taught him about writing? 'Dramatic narrative needs present tense action,' he says, almost without hesitation. He makes me laugh by describing the common tendency in apprentice playwrights to write about ancient family secrets which are revealed 'four fifths through the play, often in a drunken confessional speech.' This is 'theatrically inert' he says. Another problem is that people see life as 'something that happens to them'. It is the playwright's task, as he sees it, to change the question from 'Why is this happening to me?' to 'Why am I doing this?' It is a lesson that offers a commentary on Stephens's own work which is nothing if not immediate.

For the last 10 years or more, Stephens has been reconstituting himself as a Londoner and family man. Mile End, in East London, is home. And he says - with perhaps more excitement than the subject deserves - that he hopes to be buried in the East End: 'I adore it.' He is 'intoxicated' by the thought that he will be able to 'walk to the Olympics.'. Yet, at the same time, he condemns London for its 'atomisation' 'dislocation' and 'hostility' - an ambivalence that feeds his portrait of the city in 'Pornography'. He sees paternity (he has two boys and a girl) as vital to his writing life. 'It is fundamental. Cyril Connolly said the "pram in the hall" was the "enemy of promise". I think that is pernicious and dishonest. It has been the absolute opposite for me.' And it is possible to see tenderness sneaking into the writing. But Stephens's wife, Polly, (described as 'a full-time mum ... much harder than being a playwright') wonders at her husband's plays: 'You are quite cheery, energetic and optomistic. Why do you keep having these dark thoughts?' The closest he can come to an answer is when he says: 'I write but because I understand but because I don't. I am trying to make sense of darkness.'

Stage struck: Theatrical admirers

"Simon Stephens is wonderful, his plays have such a big heart." Laura Wade, playwright

"He writes so passionately and soulfully for ordinary people who are in really difficult predicaments. People who are violent, or whatever, can have immense humanity in them as well - Simon writes about that very well." Daniel Mays, actor

"I directed his play Port at the Exchange, and I think it's one of the best things I've ever done. His writing is so detailed, so psychologically rich, so daring in terms of his emotion. He's not very English in that way." Marianne Elliot, director

''I thought Simon Stephens's play On the Shore of the Wide World was sensational. I was thrilled when it won the Olivier because I thought it was really underrated." David Eldridge, playwright